What is Down syndrome and how does it occur?

Down syndrome (Trisomy 21) is one of the most common genetic disorders, resulting from the presence of an extra copy of chromosome 21. This condition can impact a child’s physical and intellectual development, leading to various health issues. Thanks to advances in prenatal testing, it is now possible to detect Down syndrome early during pregnancy, allowing parents to better prepare and plan for appropriate care. The following sections will discuss the causes, symptoms, and available diagnostic methods for this disorder.

Introduction to Down syndrome

Down syndrome (Trisomy 21) is a genetic disorder caused by the presence of an extra copy of chromosome 21. Human cells typically contain 23 pairs of chromosomes, totaling 46 chromosomes, which carry genetic material (DNA) and contain instructions for the growth, development, and functioning of the body. However, in individuals with Down syndrome, there is a genetic abnormality known as trisomy, where there are three copies of chromosome 21 instead of two.

The additional chromosome 21 disrupts the development of the body and brain, resulting in characteristic physical features and intellectual disabilities. This syndrome is one of the most common chromosomal disorders, occurring in approximately 1 in 700 live births.

It was first described by the English physician John Langdon Down in 1866, after whom the syndrome is named. It wasn’t until 1959 that French researcher Jérôme Lejeune discovered that Down syndrome is caused by the presence of an extra chromosome 21.

Causes and mechanism of Down syndrome

Down syndrome occurs due to the presence of an extra copy of chromosome 21. This genetic mutation typically arises during the formation of gametes (egg or sperm cells) when there is an error in cell division known as meiosis. During meiosis, chromosomes should be properly distributed, but in individuals with Down syndrome, improper segregation of chromosomes occurs, resulting in trisomy 21—an extra copy of chromosome 21. After fertilization, the embryo inherits three copies of this chromosome instead of the usual two.

This error is random and is not caused by factors such as parental behavior or external influences. In typical trisomy 21, the disorder occurs sporadically, without any genetic predisposition.

However, in the translocation form of Down syndrome, the additional genetic material from chromosome 21 can attach to another chromosome, most commonly chromosome 14. In these cases, one of the parents may be a carrier of a balanced translocation—where the chromosomes are not properly arranged, but this does not affect their health. A carrier parent can pass on this abnormality to their offspring, increasing the risk of Down syndrome in the child.

Risk factors

Certain factors increase the likelihood that a child will develop Down syndrome:

• Maternal age: The chances of giving birth to a child with Down syndrome significantly increase with the mother’s age. For example, in women aged 40, the risk is approximately 1 in 100, whereas for women aged 25, the risk is about 1 in 1,200.
• Heritability: While most cases of Down syndrome are random, the translocation form can be inherited. Parents who are carriers of a balanced translocation may have an increased risk of passing on additional genetic material to their children, which raises the likelihood that the child will develop Down syndrome.

Types of Down syndrome

There are three main types of Down syndrome:

• Regular trisomy 21: This type occurs in about 95% of cases. All children with this form of the syndrome have three copies of chromosome 21 in every cell of their body.
• Translocation: In this type, extra genetic material from chromosome 21 is “attached” to another chromosome, usually chromosome 14. Translocation occurs in about 4% of cases and can be inherited.
• Mosaicism: This rare form (about 1% of cases) occurs when only some cells in the body have three copies of chromosome 21, while others have the typical two copies. Individuals with mosaicism may show milder symptoms because some parts of the body function without genetic abnormalities.

What are the symptoms of Down syndrome during pregnancy?

Although Down syndrome does not cause physical symptoms in the pregnant mother, certain signs of the disorder in the fetus can be detected through various prenatal tests and screenings.

Prenatal tests and markers for Down syndrome:

• Ultrasound (Nuchal Translucency Test): Between the 11th and 14th weeks of pregnancy, an ultrasound can measure the thickness of fluid at the back of the fetus’s neck, known as nuchal translucency. A thicker-than-normal nuchal translucency may be an early indicator of Down syndrome.
• Biochemical tests (Double marker or Triple marker test): These tests measure the levels of specific substances in the mother’s blood, such as human chorionic gonadotropin (hCG) and the protein PAPP-A. Abnormal levels of these markers can suggest an increased risk of chromosomal abnormalities, including Down syndrome.
• Non-invasive prenatal testing (NIPT): NIPT analyzes fetal DNA circulating in the mother’s bloodstream and can accurately detect the presence of trisomy 21. This test can be performed as early as the 10th week of pregnancy and carries no risk to the mother or baby.
• Amniocentesis and chorionic villus sampling (CVS): If prenatal tests indicate a high risk of Down syndrome, doctors may recommend amniocentesis or chorionic villus sampling to collect fetal cells for chromosomal analysis. These invasive tests can provide a definitive diagnosis, though they carry a small risk of complications.

Ultrasound signs that may indicate Down syndrome

In addition to a thickened nuchal translucency, second-trimester ultrasounds can detect additional anatomical abnormalities that may indicate an increased risk:

• Short nasal bone
• Heart defects
• Enlarged kidney pelvis (hydronephrosis)
• Shortened limbs
• Increased blood flow to the liver

Symptoms and health issues in Down syndrome

Individuals with Down syndrome exhibit characteristic physical traits, symptoms, and specific health issues, which can vary in severity from person to person.

Physical characteristics

People with Down syndrome often share similar physical features, though the degree to which these traits appear can differ. Some of the most common physical characteristics include:

• Flat face and nose
• Slanted eyes with an epicanthal fold
• Short arms and legs
• Short stature
• Small mouth with a protruding tongue

Intellectual disabilities and cognitive traits

Most individuals with Down syndrome experience mild to moderate intellectual disabilities. These challenges can affect learning speed and the development of speech and social skills. However, with adequate support and education, many individuals with Down syndrome can acquire the skills necessary for independence and success in everyday activities.

Health problems associated with Down syndrome

In addition to intellectual disabilities, individuals with Down syndrome are prone to various health issues, including:

• Heart defects: Approximately 50% of newborns with Down syndrome have congenital heart defects, which may require surgical treatment.
• Vision and hearing problems: There is a higher risk of vision issues, such as cataracts, as well as hearing impairments.
• Thyroid issues: Hypothyroidism (underactive thyroid) is common in individuals with Down syndrome and requires regular monitoring and treatment.
• Respiratory issues and infections: Due to a weakened immune system, people with Down syndrome are more vulnerable to respiratory infections and conditions such as pneumonia.

These health challenges require continuous medical care and treatment to improve the quality of life for individuals with Down syndrome.

Support for people with Down syndrome

Although there is no cure for Down syndrome, various medical and therapeutic interventions can greatly improve the quality of life for individuals with this condition. Early detection and proper treatment of health issues, combined with timely support, are essential for achieving positive long-term outcomes.

Education and therapy

Children with Down syndrome can make substantial progress through special education programs designed to meet their unique needs. Inclusive education, which allows children with Down syndrome to learn alongside their peers, has been shown to be highly beneficial for their social and emotional development.

Therapies that are particularly helpful for skill development include:

• Physical therapy: Improves motor skills, balance, and coordination.
• Speech therapy: Aids in developing communication skills and improving speech.
• Occupational therapy: Focuses on building everyday skills, such as dressing and self-care, to promote greater independence.

Life expectancy and health

Thanks to advancements in medicine and improved healthcare, the life expectancy of individuals with Down syndrome has significantly increased. Today, the average person with this condition can live up to 60 years, whereas in the past, life expectancy was much shorter.

Quality of life and social integration

With the right support, individuals with Down syndrome can enjoy a high quality of life. Many become active members of society, attend school, secure employment, and some even marry and start families.

Prenatal tests for detecting Down syndrome at IntroLab

IntroLab offers a wide range of prenatal tests designed to accurately detect genetic disorders, including Down syndrome (trisomy 21). These tests utilize advanced fetal DNA analysis techniques, providing a high level of accuracy. Some of the tests available at IntroLab that can detect Down syndrome include:

• Prenatal BASIC test: A basic prenatal test that detects the most common chromosomal abnormalities, including trisomy 21.
• Prenatal PLUS test: An extended test that detects Down syndrome, as well as other chromosomal disorders such as trisomy 18 (Edwards syndrome) and trisomy 13 (Patau syndrome).
• Prenatal EXPERT test: An advanced test offering a more detailed analysis, including the detection of Down syndrome and other chromosomal abnormalities.
• Prenatal PREMIUM test: This test detects a wide range of chromosomal disorders, including Down syndrome, with high precision and provides detailed information about the fetus’s health.
• Prenatal ALL CHROMOSOMES test: A comprehensive test that analyzes all chromosomes, detecting Trisomy 21 and other disorders.

IntroLab uses the latest technologies to ensure the safety and accuracy of results, tailoring the tests to meet the specific needs of each pregnant woman.

Early detection for better preparation

Prenatal tests allow for the early detection of genetic disorders, such as Down syndrome, giving future parents essential information to prepare and adjust to their child’s specific needs. Early awareness of potential disorders helps parents plan appropriate care and support with less stress. With the expertise and reliability of IntroLab, you can count on accurate results and a smooth pregnancy experience. Contact us today to schedule your testing!